Magnetic reed contact assemblies



Nov. 11, 1969 T. HARRETT MAGNETIC REED CONTACT ASSEMBLIES Filed March 12 1968 4 Sheets-Sheet 1 Q n w T N v m I, I LL ifmwuhhi M Q. w QGRQK m RK N55) L////////// ////A/ V/ lnvenlor THOMAS HARRETT 4 Sheets-Sheet 2 Filed March 12. 1968 Inventor THOMAS HARRETT MJ tlorney Nov. 11, 1969 T. HARRETT 3, 7 7

MAGNETIC REED CONTACT ASSEMBLIES Filed March 12, 1968 4 Sheets-Sheet 5 lnvenlor THO/7A5 HARRETT 1 9 T. HARR ETT 3,478,237

MAGNETIC REED CONTACT ASSEMBLIES Filed March 12. 1968 4 Sheets-Sheet 4 Inventor THOMAS HARRETI' Claims priority, applicationGreat Britain, Mar. 17, 1967,

7 Int. or. mini 51/28, 1/66- U .S. Cl. 335-154 Claims ABSTRACT OF THE DISCLOSURE In a magnetic reed contact assembly the maximum stable voltage is increased andthe amplitude of oscillation reduced by adding an electrostatic counterbalancing.

United States Patent 0 electrode'for each reed to balance out the electrostatic forces on a reed in its normal position.

.The invention relates to magnetic reed contact as--" semblies for incorporation in magnetic reed switches.

These assemblies normally take the form of a pair of contact members of magnetic material,- one or both of which are cantilevers, sealed in a glass envelope which, commonly, is evacuated. In operation the assembly is inserted I in a solenoid or otherwise arranged so that when a magnetic field is applied across the gap between the contact members they are attracted together and'close a circuit path.

l Because of the high insulation resistance of a magnetic reed switch it might be expected that it was suitable for high speed operation in high voltage circuits, for in so far as insulation and breakdown voltage across the conto use an ordinary reed switch in a high voltage circuit, however, it is found the electrostatic attraction between 1 the contact members is such that they tend toclose without the application of a magnetic field. Although the magnetic behaviour of reed switches has been very fully investigated, so far aswe are aware attention has not previously been directed to their behaviour under electrostatic. potentials, of the order of a kilovolt or'more'. Thepresent invention is concerned with overcoming the tendency to .pre-

tact gap are concerned, a typical construction should be 'able to withstand several kilovolts. If an attempt be made mature closure at high voltages. We are: not concerned with vthe magnetic design, nor, indeed, with theproblemof using a magnetic reed switch to break a high voltage circuit under load. a

According to the present invention there. is provideda magnetic reed contact assembly including a cantilever reed of magnetic material, the .free end of which overlaps another electrode ofi-Inagnetic material and is arranged to be attracted to and contact the other electrode when a magnetic field is applied between them and to be urged to a normal, out-of contact,.position by the elasticity of the reed when the magnetic'field is removed, the contact assembly further including an electrostatic.counterbalancing electrode ofnonmagnetic material positioned opposite. and connected to the said: other electrode withthe the" reed and the counterbalancing electrode substanthe accompanying drawings in which:

FIG. 1 is a diagrammatic sketch illustratingthe feasembly according to the invention;

' tially counteracts that between the reed andthe said other 7 electrode. 1r 1 The invention will now be described with reference to Qtures ofa preferred embodiment of a double reed as- 3,478,287 Patented Nov. 11, 1969 FIG. 2 is a diagrammatic sketch for assistance in discussing the electrostatic behaviour of a conventional single reed switch;

FIG. 3 are curves illustrating the variation of elastic and electrostatic forces on the movable reed of FIG. 2;

FIG. 4 illustrates an arrangement, according to the invention, for counteracting the electrostatic force on the reed of a single reed switch;

FIG. 5 illustrates a modification of the arrangement of FIG. 4;

FIGS. 6, 7 and 8 are graphs showing the variation of the ratio of electrostatic force to elastic force in different single reed embodiments of the invention; and

FIG. 9 illustrates a further modification of the arrangement of FIG. 4 as applied to a double reed contact assembly.

In FIG. 1, where, for clarity, the parts are shown in cross-section and of much exaggerated relative thickness, a pair of leads 1 and 2 are sealed through the respective ends of an evacuated glass envelope 3. These leads are normally of magnetic material. At the end of each lead is welded a thin reed 4, 5, respectively, of magnetic material. Each reed thus constitutes a cantilever. The free ends of these cantilevers overlap one another and form the contacts of the assembly. The contacts may be plated or be provided with raised contact areas (not shown), to cooperate with one another. The parts so far mentioned are the basic elements of a double reed contact assembly, the arrangement being such that when a magnetic field is applied across the gap between the contacts, they close, and when the field is removed, they open again, being urged to normal, out of contact positions by the elasticity of the respective reeds. In the present embodiment each reed is associated with two further members, both of nonmagnetic material, brazed to the same lead 1 or 2 as the case may be. The first of these additional members is a other reed lies approximately midway between reed and counterbalancing electrode, but slightly closer to the counterbalancing electrode. The other member is a backing member 7 against which the reed rests when in its normal position. The members -6 and 7 should be substantially rigid and, to ilustrate this, are drawn much thicker than the reeds. The functions of the counterbalancing electrode 6 and backing member 7 and the ,balancing electrodes are each 0.5 inch while the reeds are 0.010 inch in thickness and separated, in their normal position, by a gap also of 0.010 inch.

Before describing further the arrangement of the present invention, let the forces beconsidered which act under high voltage operation, and in the absence of any magnetic field to close the switch contacts, in a conventional contact assembly. It will be convenient to deal with a single reed assembly such as illustrated diagrammatically in FIG. 2. Here a reed 8 is shown cantilevered from a support 9, represented as a wall, and arranged to cooperate with a rigid fixed contact member 10. Let it be assumed that the free end of the reed 8 overlaps the fixed contact member 10 by a'distance a and that, in the normal position of the reed, the opposing surfaces of the members 8 and 10 are a distance d apart. The length of the reed from the wall 9 to the middle of the overlap by the elastic restoring force f of the reed. Then, if the width of the red is b and its thickness 11 we have f1= x) ;f2= x where x=y/d, A: e ab/(2d and B=Ebh d/(4l (2) e being the permitivity of free space and E the Youngs modulus of the material of the reed. For an applied voltage V less than a certain critical voltage V the general shape of the function f, in terms of x is as shown in the curve a of FIG. 3, while that of f is the straight line b passing through the origin and intersecting the curve a at points C and D respectively. Points C and D correspond to points of equilibrium but, whereas C represents a point of stable equilibrium, at D the equilibrium is unstable. In the region between C and D the restoring force f is greater than the deflecting force 1, and so the reed, if deflected to a position between points C and D, will tend to return to the equilibrium position C. If it be deflected beyond the point D, however, the electrostatic force is greater than the restoring force and the contacts will close. As the voltage V is increased above that corresponding to curve a the equilibrium points C and D will approach one another until, at the critical voltage V the two points C and D coincide and the curve for f is tangential to the straight line b. The curve a in FIG. 3 corresponds to the variation of the electrostatic force f with proportional deflection X for V=V and is tangential to the line b at E. At E, therefore, where x=xo, Say,

ale

For a single reed contact assembly with a=0.282 cm., d=h=2.54 cms. and l=l.4l cm. (corresponding to the dimensions given in connection with FIG. 1) and with the red material having a Youngs modulus of l.72 l0 dynes/cnfl, the value of B/A is 3.37 10 volts 2 giving V =7.07 kv.

Now let the arrangement of FIG. 4 be considered. In this case, in accordance with the invention, an electrostatic counterbalancing electrode 11 of non-magnetic material is placed opposite the fixed contact member 10 so that the reed 8 is midway between them in its normal position. The electrodes 10 and 11 are strapped together so as to be at the same potential. It is evident that with the reed in its normal position the electrostatic forces are exactly counterbalanced. In this case, with the electrode 11 at a distance d from electrode 8, when the reed is deflected the expression for f the electrostatic force, becomes As previously, to find the maximum stable voltage V we equate simultaneously f and f and the first differential coeflicients of these quantities with respect to the rationalised distance It is then found that the only solution between =-l and =+1 is:

B X0*0: m In order to compare this case with that of FIG. 2, the ratio f f has been plotted in FIG. 6, on a logarithmic scale, as a function of x for both cases, and also for a third case, to be discussed below. In each case the appropriate V has been taken and the curves are for a value of V/V =)\=1. Curve a is for the arrangement of FIG. 2 and curve b for that of the balanced arrangement FIG. 4. Similar curves for )\=0.9 are shown in FIG. 7.

4 In FIG. 6 both curves at and b are tangential to the ordinate f /f ==1 and in FIG. 7 to f /f =0.81. The corresponding curves in FIGS. 6 and 7 are of the same shape, merely occupying different positions with respect to the f /f =1 ordinate.

From a comparison of curves a of FIGS. 6 and 7 it may be seen that, for Al, on application of voltage between the reed 8 and fixed contact member 10, the reed will tend to move inwards until it reaches a point of equilibrium, whose position is a function of the applied voltage, at which the electrostatic force is balanced by the elastic force on the reed. If moved beyond this point and then released, the elastic force will bring it back to the equilibrium position, provided the deflection does not exceed a critical value, corresponding to point D, beyond which the electrostatic force is the greater and causes the switch contacts to close. Increase of voltage brings the points C and D closer together until, for )\=l, they coincide, as has already been explained with reference to FIG. 3. It will be noted, however, that as A is increased, the point C moves in the direction of increasing x as the voltage is increased the reed will move inwards. Examining, now, curve b of FIG. 7, it will be seen that the point of stable equilibrium is a fixed point P at x=0. At this point the elastic restoring force is zero, for there is no deflection,

.but the electrostatic force is also zero, by reason of the balanced arrangement; the ratio of the two forces, however, remains finite with f /f l. For any deflection, to either side, up to a limiting value corresponding to the point G or a similarly positioned point on the other side of the f /f axis, the elastic force is the greater and returns the reed to its normal position; if deflected beyond G, the contacts will close. As the voltage is increased, the point G moves towards =0, but the reed remains undeflected, if initially in its normal position, until the critical voltage V is reached, whereupon it will suddenly fly over to the other contact member. Thus the arrangement of FIG. 4 not only provides a greater limting voltage than that of FIG. 2, but also an improvement in operation, for there is no partial deflection as voltage is applied.

In practice an exact balance for the FIG. 4 arrangement discussed above is unachievable. It is necessary to consider the case where there is a slight unbalance in positioning the reed in between the other contact member and the counterbalancing electrode. It will be realised at once that, when voltage is applied, there will be a position of equilibrium between the electrostatic and elastic forces slightly to one side or the other of the normal reed position, depending upon whether the counterbalancing electrode is nearer to or further from the normal position of the reed than the fixed contact member 10. Although this is not essential, we will assume, for the moment, that the counterbalancing electrode 11 is purposely placed somewhat closer to the reed, in its normal position, than is the contact member 10; The electrostatic attraction between 8 and 11 is then greater, when the reed is in its normal position and voltage is applied, than that between 8 and 10. Hence the reed will tend to move outwards. This can be prevented by the use of a rigid backing member for the reed, as shown at 12 in FIG. 5.

The arrangement of FIG. 5 differs from those previously discussed in that, as the applied voltage is increased, the reed will merely be pressed more firmly against its backing member. It is true that this implies some increase in the magnetic field which must be applied to close the contacts, but the positional unbalance need only be slight. As the reed is moved inward, the electrostatic unbalance decreases until the reed is midway between the counterbalancing electrode and the other contact member, after which its behaviour can be expected to be similar to that of the balanced arrangement.

An analysis of the arrangement of FIG. 5 may be made on somewhat similar lines to those for FIGS. 2 and 4 by substituting the following where the distance between the normal position of the reed and the counterbalancing electrode is rd, as indicated in FIG. 5. Putting V =A V where V corresponds to the maximum voltage for the balanced arrangement, a direct comparison can be made with that arrangement, though, in this case, A is not necessarily less than unity. Curves c in FIGS. 6 and 7 are graphs of 13/13 for the arrangement of FIG. 5 with r=0.9'. Only one branch of the curve has been plotted in each case. For values of x between '0 and (l-r)/2, f and f act in the same direction, namely towards =0. Since f /f has been plotted on a logarithmic scale, curve is asymptotic to =0.05 and rises steeply as x increases beyond this value. Its intersection with the ordinate f /f =1 is always at a larger value of x than that for the balanced case, r=1, for the same value of A. Thus, comparing curves b and c in FIG. 6, for values of x less than about 0.24 in the FIG. 5 arrangement the resultant force on the reed is towards the normal position. As x increases beyond this point of unstable equilibrium, curve 0 tends to merge into curve b--the behaviour of the arrangement tending to that for the balanced FIG. 4 case.

It has been'stated above that the use of a reed backing member is not essential in an arrangement as in FIG. 4 with a slight positional unbalance of the reed. If the reed in its normal position is closer to its mating contact member than to its counterbalancing electrode, one can expect a behaviour intermediate between that of FIG. 2 and the balanced arrangement of FIG. 4, with a corresponding maximum voltage intermediate between those for the FIG. 2. and balanced FIG. 4 case. If the unbalance is the other way, one may again expect to find a critical voltage and a corresponding limiting position for the reed such that, on application of slowly increasing voltage, the reed will move outwards, reaching the limiting position at the critical voltage, and then with further increase of voltage, moving over to contact the counterbalancing electrode. Putting V =N V where V is the maximum voltage in the balanced case,'r=1, it is to be expected, from what has been said above, that for any given value of r other than unity, there will be a corresponding maximum value of less than unity. In a practical case one would wish to keep high and adjust r accordingly. The situation can be analysed, using in Equations 3 the expression for h given in Equation 7. This leads to the pair of equations It can be shown that the maximum value of A satisfying these two equations is unity, and then r=1 and =0, the balanced case already discussed. For arbitrary smaller values of A, the equations can be solved numerically for r and For x chosen as 0.9, the relevant solutions of Equations 8 and 9 are: =-0.112 r=0.986 and x =0.1467, r=l.030 Graphs of the ratios f /f plotted on a logarithmic scale as functions of X for these values of A and r are shown in FIG. 8, together with the corresponding curve for r=l. As is to be expected, the curves for the two values of r, one less than and the other greater than unity, are approximately mirror images of one another with reference to =0. For voltages less than critical, the corresponding graphs would differ only by 'an upward shift of the scale of ordinates. Thus for an applied voltage at some value of X less than 0.9, the position of the ordinate f /f =l will be that indicated by the broken horizontal line, intersecting the r=0.987 curve at K, L and M and the curve for r=1.03 at R, Q and P. Consider the case where r=0.987. For values of x between -1 and (lr)/2 the electrostatic force is and outward, towards =1; the elastic force is always towards =0; thus in the small range between =0 and =0.006-5 both forces act in the same direction, namely towards =0. Between =0.0065, and 0.37, corresponding to the point M, f /f 1. Hence for deflections between the points L and M the resultant force is such as to tend to restore the reed to the position corresponding to point L. Similarly the reed released from a position between K and L will move towards L. If the reed were held towards the counterbalancing electrode beyond K, or be moved inwards beyond M, on release it would fly to the counterbalancing electrode or the other contact member, respectively. The rest position is at L. If now, with the reed resting at L, the voltage is increased, the reed will move outward until, at the critical voltage, it arrives at the value of ='0.l12 If the critical voltage is exceeded, it will fly over to the counterbalancing electrode. The behaviour, generally, is thus similar to the uncompensated case of FIG. 2 with the reed moving outward instead of inward as applied voltage is increased. For r 1, the reed moves in the same direction as in the arrangement of FIG. 2 when voltage is applied. However, with the counterbalancing electrode the degree of such movement is very small indeed compared with that of the FIG. 2 arrangement, while the critical voltage, in the present example where A =0.9, is some 12% higher than in the uncompensated arrangement.

The above analysis has been made purely from static considerations. In the event that, before any magnetic field is applied, the switch operating voltage is suddenly applied, with a contact assembly according to the invention having a backing member, the reed will be pressed against the backing member by the electrostatic forces and will remain stationary. In the case, however, where no backing member is provided, the reed will be attracted towards its equilibrium position and will tend to overshoot this position. The differential equation of the motion is where m is the inertial mass of the reed. Multiplying both sides by Zd /dt and making the substitutions the velocity v of the reed initially at rest in its normal position, is given by If there is a real value of v for all X Within the range being considered, the overshoot will continue and the reed will move over to close with its other contact member or to contact the counterbalancing electrode, as the case may be. If the overshoot is limited, then v will not be real for values of X beyond that corresponding to the amount of overshoot. Now for r l and 0 1 and for r 1 and r 0, z 0, so that v is real for all values of x in the range considered if 1 +1" (13) where z is the maximum value of z. This means that if the applied voltage exceed a certain critical value, the reed will commence to move from its normal position and continue to move until it meets its other contact member or the counterbalancing electrode, depending on whether the normal position of the reed is closer to the other contact member or to the counterbalancing electrode, respectively. For lower applied voltages, v will be imaginary over part of the range, which means that the reed will stop and then reverse its direction of motion, eventually coming to rest in its equilibrium position by reason of damping, which we have not considered in the differential equation. Thus Equation 13 defines the maximum applied voltage for stability under dynamic conditions. For a value of r= 0.987, Equation 13 yields a value of A =O.881 at a value of x=0.l94 For 1 :1.03, A 0.875 at =0.23 The critical dynamic voltage is thus, as is to be expected, somewhat less than the maximum static voltage for stability and its application causes the reed to deflect to somewhat beyond the position of limiting static stability before reversing its motion. Since, at its maximum deflection under dynamic conditions, the reed is momentarily stationary, the position of maximum deflection is that of the corresponding limiting deflection, at the reduced voltage, in the static case.

For the case of the balanced reed, r=l, there can be no deflection on application of voltage so that the maximum voltage under dynamic conditions is the same as that for static conditions. In practice there must always be some slight unbalance in positioning the reed, so that case is only of academic interest.

.For the arrangement of FIG. 2, with no counterbalancing electrode, the relationship between maximum voltages under dynamic and under static conditions is dynamic= statlc and the reed may swing half way towards the fixed contact member. Thus embodiments of the present invention show improved dynamic as well as improved static characteristics as compared to the conventional reed assembly.

Before leaving this subject, mention should be made of double reed switches according to the invention, dif fering from the embodiment of FIG. 1 in that backing members for the reeds are omitted. The analysis of this case is similar to that given above for the single reed, and analogous results are obtained. The principal difference in the analysis is that, since both reeds move equally, the value of x in terms involving electrostatic attraction between the reeds is doubled. This leads to corresponding values of maximum voltage being l/V times the corresponding values in the equivalent single reed case. Thus, for the various arrangements discussed V (single reed)=2V (double reed) whether conventional arrangements or those according to the invention are being considerd. In particular, the arrangement of FIG. 1 is basically that of FIG. 5 except that, in place of one fixed contact member and one movable reed, there are two movable reeds each similarly provided with its own counterbalancing electrode and its own backing member. If desired the backing members could be omitted; the numerical design values would differ somewhat from those given above, but the general behaviour is similar.

In the embodiments discussed above it has been assumed that the counterbalancing electrode is of the same efiective length as its reed. This is not essential; it would in fact be possible, instead of making a gap between a reed and its counterbalancing electrode smaller than that between the reed and its fellow contact member, to increase the overlap of the counterbalancing electrode and so increase the outward electrostatic force.

Another arrangement for increasing the spacing between a reed and its counterbalancing electrode is shown in FIG. 9. Here dielectric material 13 is introduced between the reed 8 and its counterbalancing electrode 11. The dielectric material 13 may conveniently be attached to the counterbalancing electrode 11 and also serve as a backing member for the reed. FIG. 9 shows a double reed arrangement with a second reed 8a replacing the fixed contact member 10 of FIGS. 4 and 5. The reed 8a and the counterbalancing electrode 11 are both supported from a wall 9a and the reed 8a is arranged similarly to the reed 8 with a counterbalancing electrode 11a electrically connected to reed 8 through wall 9 and carrying dielectric material 13a interposed between it and its cooperating reed 8a. If the dielectric constant be k, the presence of the insulating material increases the outward com ponent of electrostatic force on each reed by the factor k. Therefore to equalise the outward and inward components (at zero deflection) the outer gaps may be increased by the factor k Thus if the material used were mica, of dielectric constant 5, say, the outer gaps would be 2.24 X the inter-reed gap, with the consequent advantage of lower electric intensity at the cost of some lowering of insulation resistance due to the presence of the solid dielectric.

It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation on its scope.

I claim:

1. A high voltage magnetic reed contact assembly including a cantilever reed of magnetic material the free end of which overlaps another electrode of magnetic material and is arranged to be attracted to and contact the other electrode when a magnetic field is applied between them and to be urged to a normal, out of contact, position by the elasticity of the reed when the magnetic field is removed, the contact assembly further including an electrostatic counterbalancing electrode of non-magnetic material position opposite and connected to the said other electrode with the free end of the reed in between them, the arrangement being such that when a high voltage is applied between the reed and the said other electrode, the reed being in its normal position, the electrostatic attraction between .the reed and the counterbalancing electrode substantially counteracts that between the reed and the said other electrode.

2. An assembly as claimed in claim 1 wherein the spacing between the free end of the reed, in its normal position, and the counterbalancing electrode is less than that between the reed and the said other electrode.

' 3. An assembly as claimed in claim 1 wherein dielectric material is interposed between the counterbalancing electrode and the reed.

4. An assembly as claimed in claim 1 including a rigid non-magnetic backing member against which the free end of the reed rests when in its normal position.

5. An assembly as claimed in claim 1 wherein the said other electrode is a second cantilever reed and a further counterbalancing electrodes is provided for the second reed, the arrangement of the second reed and its counterbalancing electrode being substantially as specified for the first mentioned reed.

References Cited UNITED STATES PATENTS 2,450,499 10/1958 Brown 33S-l54 2,922,856 1/1960 Karrer 335-l54 BERNARD A. GILHEANY, Primary Examiner R. N, ENVALL, JR., Assistant Examiner 

